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Currents on coaxial cable

  1. Aug 21, 2015 #1
    Hello Forum,

    A coaxial cable is a two conductor transmission line.

    If we connect a DC voltage source via a coaxial cable to a resistor so that the positive terminal is connected to the inner conductor and the negative terminal to the shield , the current will flow through the whole cross-section of the inner conductor, go through the resistor and return to the source through the entire thickness of the outer conductor (called shield). What if the negative terminal was connected to the inner conductor instead? Any significant difference?

    Also, we could ground either the inner or the outer shield conductors...

    If the source was AC instead of AC, the currents will behave the same way, flowing on both conductors, back and forth. But I think I read the shield current will only flow on the inner surface of the shield....do you know why?

    Is it better to ground the inner conductor or the shield of the coaxial cable? The shield is always grounded but I am not sure why....

    Grounding is a complex topics with many different configurations....it can be done for safety or for preventing EMI...

    thanks,
    fog37
     
  2. jcsd
  3. Aug 21, 2015 #2
    There is no circuit critical difference about which side is positive, shield or conductor. There are concerns like safety and grounding issues obviously. Either could be grounded, but since people are far more likely to touch the shield, it would be dangerous to ground the conductor with significant voltage/power in the cable.

    Yes I know why the current stays near the surface. It's called the skin effect. Current flows in transmission lines to support the electromagnetic fields between the conductors. This is why the current likes to stay on the inside -- that's where the fields are.

    Preventing EMI is an issue. The skin effect makes the shield protect the conductor (to some extent). Stray signals have trouble getting through. Both this and safety are good reasons to ground the shield. Still, safety first.
     
  4. Aug 22, 2015 #3

    davenn

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    Gold Member

    No, the RF current flows on the outer part of the inner conductor ... the skin effect that Jeff spoke of
    he may have mis-read your comment and not seen what your wrote

    Having RF currents flowing on the shield is a bad thing and is to be avoided
    this is because the shield will act as a radiator and will produce EMI

    The RF energy flows as an EM wave within the dielectric between the inner conductor and the shield.
    A coax cable can be though of as a type of waveguide transmission line where the only difference between it and a standard waveguide (a hollow tube )
    is that a the standard waveguide has done away with the centre conductor


    Dave
     
    Last edited: Aug 22, 2015
  5. Aug 22, 2015 #4
    You are right. I misread that.

    The RF current does flow primarily on the inner side of the shield, but it also flows on the outer edge of the inner conductor. The fields form between the two conductors with the currents on their surfaces guiding the fields.

    If you read the article on skin effect, you would notice the current goes some depth into the conductors. The depth it goes depends on the frequency.
     
  6. Aug 22, 2015 #5
    Ok, thank you.

    I understand how energy travels in the form of a TEM wave from the source to the load in the dielectric between the two conductors. But currents exists on both conductors.
    For the DC case: there is a current in the inner conductor and a "return" current on the outer conductor on its inner surface (not on the outer surface). Any current on the top surface is bad....

    That said, I am not sure why the return current on the outer conductor would flow on its inner surface only...
     
  7. Aug 22, 2015 #6
    It's the skin effect.

    Basically the TEM wave sets up eddy currents which block it from traveling deeper into the conductor. These eddy currents resist changing fields, but damp down near DC. Thus DC uses the whole wire, including the outside.
     
  8. Aug 24, 2015 #7
    Hello,

    I think I understand the skin effect: at frequencies >0, current prefers to follow in that part of a conductor closer to the edges. That said, in the case of the coaxial outer conductor (the shield), current should be both on its inner and outer surface based on how the skin effect work. But we only find a current on the inner surface of the outer conductor facing the cylindrical inner conductor...

    thanks,
    fog37
     
  9. Aug 24, 2015 #8
    The current forms in response to the electric field. The field is contained between the conductors.
     
  10. Aug 25, 2015 #9
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